Labelling machines for heat-shrinkable sleeve labels are known and used for labelling containers or objects of various shapes with a label that wrap them up completely. Heat-shrinkable sleeve labels are particularly suitable in the case of containers or objects of very irregular shapes, because they are able to fit these shapes exactly during the heat-shrinkage step.
These labelling machines are of two main categories. The first type provides for a continuous reel of flattened tubular labels, which are cut at the appropriate length and are then opened just before wrapping them around a container. The containers with their own sleeve labels are passed through a suitable oven wherein the heat provides for shrinking the sleeve label on the container. The second category comprises machines wherein the sleeve is not pre-formed, but is created from a roll film of labels that are cut, wound around a mandrel to overlap their leading and trailing ends, welded or glued to form the sleeve label and finally transferred onto the object to be labelled before the shrinking step.
In the labelling machines of this second type, commonly the welding is performed by means of ultra-sound heads, one for each container/mandrel unit, that are positioned on the carrousel carrying the mandrel/container units, internally with respect to the circle delimited by the containers so positioned on the carrousel. Each ultra-sound head is mounted on a frame that is moved, by means of electrical and/or pneumatic actuators, forward and backward to approach or distance the corresponding mandrel and, hence, the sleeve label in place thereon. During the welding operation, when the ultra-sound head is close to a sleeve wound on a mandrel, it is moved vertically in order to weld together the overlapping ends of the label, thus forming the sleeve.
This system has however some disadvantages. First of all, the provision of a plurality of ultra-sound heads, each mounted on a frame that performs a complicate motion, can be subject to frequent damages and causes long and difficult maintenance operations. Additionally, the quality of the welding is not optimal, as it can be uneven or not linear or not perfectly vertical.